Florida Rep’s “The Unexpected Guest” opens on a room plunged in shadow. Moonlight filters through the roiling fog outside. A woman lurks quietly by the bookcase and trembles in the darkness.

Near the center of the room, a man sits in a wheelchair with his back turned to the audience. He doesn’t move.

Soon a flashlight from outside shines into the room and a mysterious stranger — the unexpected guest, naturally — knocks on the study’s door-like double windows.

Things get really interesting after that.

This is a murder mystery, so of course the man in the wheelchair turns out to be dead — and it sounds like he had it coming, too.

The trembling woman, the dead man’s wife, claims she’s the one who shot him in the head (but really, almost everybody on this isolated South Wales estate has a reason to kill the guy). And the unexpected guest quickly gets involved in the cover-up for no other reason, it seems, than it’s fun.

I’m not a huge fan of murder mysteries, but director Robert Cacioppo and his Florida Rep cast managed to keep me entertained and guessing in this whodunit involving a cruel murder victim (a famous big game hunter with a drinking problem and a sadistic streak), a long-ago car crash that left a child dead, a mysterious cigarette lighter, a blackmail attempt, a fired gardener and more.

Cacioppo and company do a great job creating mystery and tension here, from that atmospheric opening to the creepy ambient music underscoring key scenes to a masterfully staged twist ending that comes just as you think you have it all figured out.

There are plenty of other red herrings and twists along the way. It’s Agatha Christie, after all.

Florida Rep’s strong cast includes Margaret Loesser Robinson as the dead man’s attractive wife; Britt Michael Gordon as his fidgety, mentally deficient younger brother; Larry John Meyers as a gruff, no-nonsense police inspector; and especially David Breitbarth as a cold-eyed, calculating and deadly still valet that all but screams “suspect.”

But it’s Lou Sumrall that impresses the most as unexpected guest Michael Starkwedder, who crashes his car in a ditch on this foggy night, wanders into the murder scene and, oddly, starts helping the freshly made widow come up with a cover story. Why? “I always had a secret longing to see how I could get on in a detective story,” he explains later.

Sumrall is lots of fun as he gleefully dives into the long, expertly paced opening scene with Robinson. You can see the machinery of his brain working as he wipes brandy glasses clean of fingerprints and manufactures fake evidence. I wish Sumrall was in more scenes, in fact, but he makes the most of the ones he has — including a smoke break in Act 1 that says so much without using any words.

I should also point out Bert Scott’s impressive set: A manly study full of leather furniture, wood paneling, books and mounted animal heads. And the moon-lit fog outside the window is almost a character, itself, swirling and writhing like a living thing.

Another fun thing about this production: At intermission, audience members get to cast ballots to vote on who they think did the deed. “Are you ready to be sleuths tonight?” Cacioppo asks during his curtain speech. Apparently, they are.

The show isn’t perfect. At about 2 ½ hours, including intermission, I wish it was a bit shorter. Again: Not a big fan of murder mysteries.